In any microwave signal generator that employs an yttrium-iron-garnet ("YIG") or other ferri-resonant oscillator material, the microwave output signal frequency is proportional to the current flowing in the oscillator coil. The output signal frequency can be shifted quickly by shifting the current level quickly.
Kaye et al., in U.S. Pat. No. 4,536,722, observes that a capacitive filtering means is usually required across the input terminals of a YIG oscillator to suppress noise and similar problems. The presence of the capacitive filter introduces time lag and response delay problems and ringing, where the frequency is to be changed quickly. Kaye et al. provide switching means for selectively connecting the capacitive filtering means across the input terminals and disconnecting the capacitive filtering means at times when the frequency is to be changed. The capacitive element is charged to its new value before the filtering means is reconnected. Protection of the YIG oscillator during these times of change is not discussed in this patent.
In U.S. Pat. No. 4,581,594, Kennedy discloses a low noise drive circuit for YIG-tuned oscillators in which a feed-back loop containing the oscillator coil is selectively activated or deactivated in order to control the tuning current passing through the oscillator coil.
What is needed here is a low-noise current driver for the oscillator coil that operates in a low noise first state when the current input is stable and switches to a second state which allows fast settling when a transition occurs from one current level to another current level. The drive circuit should manifest low noise at all relevant frequencies in the first, stable state, and should permit switching from one current level to another at times on the order of 1-20 msec in the second state. The current driver must be made with readily-available and reliable parts (low failure rate). It is preferable that the circuit not require current surges from power source during current level transitions. Preferably, the current driver should be capable of withstanding any transient voltage levels which will occur during current level transitions.